全文获取类型
收费全文 | 252048篇 |
免费 | 3231篇 |
国内免费 | 995篇 |
专业分类
化学 | 137095篇 |
晶体学 | 3773篇 |
力学 | 10368篇 |
综合类 | 5篇 |
数学 | 28261篇 |
物理学 | 76772篇 |
出版年
2021年 | 1959篇 |
2020年 | 2117篇 |
2019年 | 2449篇 |
2018年 | 2909篇 |
2017年 | 2754篇 |
2016年 | 4399篇 |
2015年 | 2999篇 |
2014年 | 4438篇 |
2013年 | 12169篇 |
2012年 | 8835篇 |
2011年 | 10696篇 |
2010年 | 7090篇 |
2009年 | 6885篇 |
2008年 | 9437篇 |
2007年 | 9520篇 |
2006年 | 8650篇 |
2005年 | 8107篇 |
2004年 | 7161篇 |
2003年 | 6376篇 |
2002年 | 6271篇 |
2001年 | 7271篇 |
2000年 | 5406篇 |
1999年 | 4275篇 |
1998年 | 3583篇 |
1997年 | 3627篇 |
1996年 | 3331篇 |
1995年 | 3160篇 |
1994年 | 2982篇 |
1993年 | 3013篇 |
1992年 | 3326篇 |
1991年 | 3357篇 |
1990年 | 3170篇 |
1989年 | 3131篇 |
1988年 | 3187篇 |
1987年 | 3059篇 |
1986年 | 2949篇 |
1985年 | 4013篇 |
1984年 | 4178篇 |
1983年 | 3417篇 |
1982年 | 3774篇 |
1981年 | 3661篇 |
1980年 | 3534篇 |
1979年 | 3568篇 |
1978年 | 3793篇 |
1977年 | 3618篇 |
1976年 | 3824篇 |
1975年 | 3399篇 |
1974年 | 3528篇 |
1973年 | 3829篇 |
1972年 | 2338篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Principles and recent developments in ultrasound contrast agents 总被引:6,自引:1,他引:5
The behaviour of gas bubbles and gas encapsulated spheres as echographic contrast agents is reviewed. Compared with rigid spheres, gas bubbles are superior scattering agents and they offer a number of useful properties which can be exploited in a variety of ways. The analysis of their velocity of sound, back-scatter intensity, second harmonic emission and resonant frequency opens up new perspectives in the development of contrast agents for echocardiographic research with potential clinical applications. 相似文献
993.
New concepts for the study of incompressible plane or axisymmetric flows are analysed by the stream tube method. Flows without eddies and pure vortex flows are considered in a transformed domain where the mapped streamlines are rectilinear or circular. The transformation between the physical domain and the computational domain is an unknown of the problem. In order to solve the non-linear set of relevant equations, we present a new algorithm based on a trust region technique which is effective for non-convex optimization problems. Experimental results show that the new algorithm is more robust compared to the Newton-Raphson method. 相似文献
994.
A general class of Fuller modified maximum likelihood estimators are considered. It is shown that this class possesses finite moments. Asymptotic bias and asymptotic mean squared error are derived using small-σ expansions. A simulation study is carried out to compare different estimators in this class with standard estimators. 相似文献
995.
996.
997.
998.
M.N. Kotby S.R. El-Sady S.E. Basiouny Y.A. Abou-Rass M.A. Hegazi 《Journal of voice》1991,5(4):316-320
The management of voice disorders may follow one of four approaches: surgical, pharmacological, technical aid, and/or voice therapy. The latter approach offers a wide variety of techniques, each based on certain theoretical beliefs. One of these techniques is the Smith Accent Method. The principles and goals of the Accent Method are presented. The results of its application are discussed. This technique proved effective for cases of functional voice disorders, vocal nodules, and some degrees of vocal fold paralysis. 相似文献
999.
E. Alfinito M. Leo R. A. Leo M. Palese G. Soliani 《Letters in Mathematical Physics》1994,32(3):241-248
The Estabrook-Wahlquist prolongation method is applied to the (compact and noncompact) continuous isotropic Heisenberg model in 1 + 1 dimensions. Using a special realization (an algebra of the Kac-Moody type) of the arising incomplete prolongation Lie algebra, a whole family of nonlinear field equations containing the original Heisenberg system is generated. 相似文献
1000.
Thomas J. Cloonan Gaylord W. Richards Anthony L. Lentine Frederick B. McCormick John R. Erickson 《Optical and Quantum Electronics》1992,24(4):S415-S442
Free-space optical implementations of switching networks have been proposed to circumvent many of the system-level problems that may be encountered in systems that require many high-density, high-bandwidth connections. The details of a new class of switching network (the EGS network), that is well-suited to free-space implementations, is described. The common control injection problem that plagues most free-space photonic networks, i.e. how can control information from an electronic source be injected into the network for applications that require relatively high network reconfiguration rates, is examined. A new technique for control injection, called embedded control, which permits network operation even with relatively high network reconfiguration rates is also proposed. 相似文献